1. Field of the Invention
The present invention relates to a method of improving the uniformity of waveguides formed in optical substrates and, more particularly, to a method wherein the optical substrate is pre-annealed to form a surface layer with a predetermined composition.
2. Description of the Prior Art
Integrated optical components formed on optical substrates are becoming increasingly prevalent in the field of lightwave communications and will become even more so in the future. These components most often utilize optical waveguides diffused into the substrate as part of the active device structure. In particular, optical waveguides usually comprise strips of titanium which are diffused into a lithium niobate optical substrate, although other materials may be used for both the waveguides and the substrate. The formation of this type of waveguide structure is disclosed in U.S. Pat. No. 4,284,663 issued to J. R. Carruthers et al on Aug. 18, 1981. As disclosed, a titanium waveguide is formed by selectively indiffusing metal ions into the crystal to effect an increase in the index of refraction of a surface portion of the crystal. The indiffused metal ions serve to raise one or both of the ordinary and extraordinary (n.sub.o and n.sub.e) indices of the top layer of the substrate, where by properly adjusting the relevant diffusion parameters the waveguide characteristics of the surface may be controlled to provide the necessary guiding.
The diffusivity properties of titanium as related to lithium niobate have been studied in order to improve the operation of integrated optical components. One such study, entitled "Titanium diffusion into LiNbO.sub.3 as a function of stoichiometry", by R. J. Holmes et al and appearing in the Journal of Applied Physics, Vol. 55, No. 10, May 1984 at p. 3531-5, involved the measurement of Ti diffusivity into LiNbO.sub.3 as a function of crystal orientation (x,y or z-cut crystal) and Li/Nb ratio. The study concluded that enhanced lateral diffusion (control of which is necessary to obtain mode confinement) would result from forming an Li.sub.2 O outdiffused layer on top of the crystal. This would give, in effect, a depth dependent diffusion coefficient.
A limitation which currently exists in the formation of integrated optical devices is the difficulty of obtaining reproducible performance from like devices fabricated on different substrates. A major source of this difficulty relates to the boule-to-boule compositional variation in lithium niobate crystals, where it has also been shown that the crystal composition may vary along the length of a given boule. Thus, a need remains for a way of assuring that titanium diffusivity is independent of the particular optical substrate being employed.